Micromechanical Properties of Duplex Microstructure of Martensite/Bainite in Hot Stamping via the Reverse Algorithms in Instrumented Sharp Indentation
ZHU Bin, YANG Lan, LIU Yong(), ZHANG Yisheng
State Key Laboratory of Materials Processing and Die & Mould Technology, School of Materials Science and Engineering, Huazhong University of Science and Technology, Wuhan 430074, China
Cite this article:
ZHU Bin, YANG Lan, LIU Yong, ZHANG Yisheng. Micromechanical Properties of Duplex Microstructure of Martensite/Bainite in Hot Stamping via the Reverse Algorithms in Instrumented Sharp Indentation. Acta Metall Sin, 2022, 58(2): 155-164.
Lightweight automobiles have a lower impact on the environment and save energy; therefore, they have become a focus within the automobile industry. Hot stamping parts made of high-strength steel have been widely used in car bodies. To study the mechanical properties and constitutive model of high-strength steel after hot stamping, the samples containing full martensite, full bainite, and martensite/bainite dual phases structure were obtained by controlling the tool's temperature and holding time during hot stamping. Then the load-displacement curves of different microstructures were obtained using nanoindentation tests. Subsequently, the modulus of elasticity, yield stress, strain hardening exponent, and other mechanical properties of these microstructures were calculated by reverse algorithms using dimensional analysis. Further, the power-law elastoplastic constitutive models of different microstructures were derived using these parameters. The errors of yield strength obtained using the reverse algorithm and tensile tests in full martensite and full bainite samples are -1.15% and 3.38%, respectively. The yield strength of the martensitic/bainite sample obtained using the reverse algorithm is 16.62%, 24.17%, and -11.78% different from that obtained by the tensile test, showing that the mechanical properties are different under macroscopic and microscopic conditions to some extent. Simultaneously, the average yield strength of the three points is only -1.41% different from that obtained using the tensile test. Finally, the derived constitutive models were verified by simulating the finite element nanoindentation. The results show that the constitutive model obtained using the inverse algorithm can accurately describe the mechanical properties of the main microstructures of high-strength steel after hot stamping.
Fig.2 Cooling curves of the samples under different tools temperatures and holding time during stamping (A: austenite, B: bainite, F: ferrite, P: pearlite; M—martensite, Ms: martensite transformation starting temperature, Mf: martensite transformation finishing temperature, CCT—continuous cooling transformation)
Fig.3 SEM images of microstructures of the samples under different tools temperatures and holding time during stamping
Fig.4 Load-displacement curves of the three representative nanoindentation points (Nos.1-3)
Fig.5 SEM images of microstructures of different nanoindentation points
Fig.6 Load-displacement curve of the nanoindentation test (h—depth,P—load,hm—maximum indentation depth, Pm—maximum load, hr—residual indentation depth, C—loading curvature, Wp—plastic work, We—elastic work, Pu—unloading force, —initial unloading slope)
Indentation
E / GPa
σy / MPa
n
No.1
212.69
770.2
0.098
No.2
215.96
1147
0.102
No.3
211.58
814.9
0.104
Table 1 The results of the inverse analysis algorithms
Fig.7 Stress-strain curves of the specimen (400oC, 30 s) and three different indentation points calculated by the inverse algorithms in nanoindentation under the same process conditions
Fig.8 Stress-strain curves of full martensite (cold tool, 30 s) and full bainite (450oC, 240 s) specimens, and the stress-strain curves of the same structures obtained by the inverse algorithms in nanoindentation
Fig.9 Finite model of nanoindentation
Fig.10 Stress distributions in simulation
Fig.11 Load-displacement curves in tests and simulations at different indentation points
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